1. Field of the Invention
The present invention relates to the production of polymer gasoline from streams containing C.sub.3 to C.sub.4 alkenes, in particular normal C.sub.3 to C.sub.4 alkenes, more specifically the invention is a process for producing polymer gasoline using an acid cation exchange resin and a catalyst modifier.
2. Related Art
Refinery streams having mixtures of C.sub.3 and C.sub.4 hydrocarbons, in particular large amounts of normal olefins are not of particular value, since the normal C.sub.3 and C.sub.4 olefins are not desirable gasoline components and their separation from the other components, such as the normal butane which is used in gasoline blending, by fractionation requires substantial equipment and energy.
There are several existing processes for contacting these streams with fixed bed catalysts to cause oligomerization of the normal olefins to produce streams of C.sub.7 to C.sub.8 olefins.
These processes, either because of the nature of the olefin product or nature of the catalyst (expense or special handling required) have not gained full industry acceptance. For example the phosphoric acid oligomerization of normal butene typically produces substantial amounts of the trimethyl pentenes which are not desirable as gasoline components.
The reaction of olefins to form longer chains of two or more monomer units is well known. It is well known that ethylene and other olefins can be polymerized to form relatively low molecular weight products through the use of certain organo-metallic catalyst. Broadly, these catalysts are compounds of metals of groups IV to VI of the Periodic Table of elements in combination with other compounds of metals of groups I to III. The Ziegler catalysts are representative of such catalysts and are preferably specific combinations of titanium halide and a trialkyl aluminum component, with or without other metal promoters. Other catalysts such as alkyl aluminum halides (preferably the chloride) in combination with alkyl titanium esters have also been used to carry out this reaction.
These catalysts are homogeneous in that they are soluble in the reaction medium. The catalysts are very effective, that is the reaction can easily be carried out to produce high molecular weight compounds, with the appropriate amounts of catalyst. In some instances, by using extremely low concentrations of catalyst in the reaction system, low molecular weight products may be produced, in particular dimers, trimers and tetramers.
Free radicals, carbonium ions and carbanions have also been used to promote the reaction of olefins, particularly alpha-olefins to produce polymers of high molecular weight.
The use of acid catalysts such as sulfuric acid to selectively react isoolefins in the presence of other olefins to produce lower molecular weight products, e.g., U.S. Pat. Nos. 3,546,317 and 3,832,418 is also well known. U.S. Pat. No. 4,065,512 discloses the use of perfluorosulfonic acid resin, particularly in the form of films for the polymerization of isobutene to produce dimers, trimers and higher oligomers. U.S. Pat. No. 4,071,567 discloses a two stage process for reacting methanol and isobutene, wherein an excess of isobutene is present in the second stage which results in isobutene dimerization.
UK Pat. Spec. No. 973,555 discloses the oligomerization of n-butene in an autogenous slurry. U.S. Pat. Nos. 4,215,011 and 4,242,530 disclose the use of acid cation exchange resin in a heterogenous combination reaction-distillation system for the selective dimerization of isobutene in the presence of normal butenes. The reaction is highly preferential for the reaction of isobutene with itself, although some codimer between n-butenes and isobutene are formed, and provides a means to separate isobutene from a C.sub.4 stream.
UK Pat. specification No. 2,086,415 B discloses a process for the liquid phase oligomerization of C.sub.2 to C.sub.10 normal olefins using a fixed bed acid cation exchange resin.
U.S. Pat. No. 4,232,177 disclosed the production of diisobutene in a catalytic distillation by reaction isobutene in the presence of less than stoichiometric amounts of methanol at increased temperatures to favor production of the dimer over MTBE.
U.S. Pat. No. 4,375,576 discloses that in the liquid phase, for the reaction of isobutene with itself to form dimer in a stream containing n-butenes, the presence of 0.0001 to 1 mole of methyl tertiary butyl ether (MTBE) per mole of alkene, suppresses the formation of higher oligomers and codimers of isobutene and normal butenes, to maximize diisobutene production and n-butene recovery.
In carrying out the reaction of normal C.sub.2 to C.sub.10 olefins, particularly the C.sub.3 and C.sub.4 normal olefins using cation exchange resins, good results have been achieved as disclosed in commonly assigned U.S. patent application Ser. No. 425,106 filed Sept. 27, 1982, now U.S. Pat. No. 4,463,211, which is incorporated herein; however, catalyst life has been found to have been relatively short. The normal course of events being an observed decline in conversion at a given temperature, requiring increase of the temperature to maintain the conversion. However, increasing the temperature of the bed also shortens the catalyst life, for example, by desulfonating the sulfonated resins. It is believed that the initial drop in conversion is the result of polymer (higher oligomers) which is formed in the reaction, plugging the catalyst pores. The present invention has found that a small amount of a material which appears to function as a cosolvent to dissolve and remove the polymer, extends the life of the catalyst by allowing operation of the process at lower temperatures (at least at a slower rate of temperature increase) and improves the selectivity of the oligomerization to n-butene dimer.